During operation of engines employing fans, the rotating fan blades, which are mounted to a rotor, are subjected to forced vibration due to oscillatory excitation at frequencies that coincide with integral multiples (harmonics) of the rotor's rotational frequency. Such excitation is often referred to as synchronous excitation. Synchronous blade excitation is typically created by non-uniformities in the flow of the motive fluid that may vary in the space around the circumference of the fan. Such non-uniformities may result from imperfections in the shape and spacing of the fan duct or supporting structures proximal to the fan intake. As a result of synchronous excitation, fan blades undergo high frequency deflections that create vibratory stresses. These vibratory stresses can result in high cycle fatigue cracking if their magnitude is not controlled. This problem is exacerbated by the fact that a fan blade typically has a number of resonant frequencies associated with its various vibratory modes. If the frequency of the oscillatory excitation to which the blade is subjected is close to one of its resonant frequencies, the vibratory stresses can quickly build up to destructive levels. This occurrence can be avoided if the rotating blade has a natural resonant frequency at or near the midpoint between two successive harmonics of the rotor's rotational frequency. Generally, within a given blade row, the airfoil shape of each of the blades is identical, within manufacturing tolerances. This configuration is sometimes referred to as "tuned.
In contrast to forced vibration, a complex aeroelastic phenomenon known as flutter may occur even if the blades are properly tuned between two harmonics. Briefly, flutter is an aeroelastic instability wherein vibratory deflections in the airfoil cause changes in aerodynamic loading that tend to increase, rather than dampen, the deflections. Consequently, flutter can increase the vibratory stress on the blade and cause high cycle fatigue cracking. Flutter may occur when two or more adjacent blades in a blade row vibrate at a frequency close to their natural resonant frequency and the vibratory motion between the adjacent blades assumes a certain phase relationship. One solution proposed in the past for increasing the resistance of turbine blade rows to flutter is to form the row using blades of varying frequency, a method referred to as "mis-tuning." The mis-tuned blades are installed in the blade row so that each blade alternates with another blade having a slightly different resonant frequency. Such mis-tuning makes it more difficult for the blades to vibrate at the same frequency, thereby inhibiting stall flutter. However, the increased number of blade natural frequencies increases the bandwidth of the blade responses to forced vibrations.
From the foregoing, it can be observed that the conventional methods of reducing forced vibratory response tend to promote flutter, and vice versa. One method claiming to prevent self-excited vibration between adjacent rotor blades without increasing the effects of forced vibration involves profiling or shortening the tips of half of the blades in the row in order to modify their resonant frequency, as discussed in U.S. Pat. No. 4,878,810 to Evans ("'810 patent"). Unfortunately, tip profiling creates compound leading edges and shortening the blade increases losses associated with blade tip clearance. Each of these tip modifications therefore undermines the aerodynamic performance of the blade.
Accordingly, what is needed is an improved propulsion engine fan configuration that eliminates flutter while reducing destructive resonant response, without material adverse effect on the aerodynamic performance characteristics of the engine.